Recent years have seen remarkable technological advances in the lightening and downsizing of semiconductor packages, and many package structures have been proposed and brought to market. Particularly noteworthy are area-mounted formats in which a semiconductor package is bonded to the circuit board (motherboard) by means of bump electrodes of solder or the like, instead of by conventional lead frame bonding.
In particular, the flip-chip format in which bump electrodes are directly provided on the circuit surface of a semiconductor chip is one method of minimizing packages. Flip-chip mounting, in the case of solder electrodes, involves treating the surface of the solder electrodes with flux to remove oxide layers, followed by bonding by reflow or the like. As a result, flux can remain in the areas around the solder electrodes and circuit board and cause problems in the form of impurities. Therefore, encapsulation is performed after a cleaning treatment to remove the flux. The reason for this is that the semiconductor package is directly bonded to the circuit board (motherboard) by bump electrodes, so when reliability tests such as temperature cycling tests are performed, differences in the coefficient of thermal expansion between the chip and the circuit board can cause electrical defects in the electrode junction portions.
The above-described semiconductor package encapsulation involves applying an encapsulating resin on one side or multi-sides of the chip and allowing the resin to seep into the gaps between the circuit board and the chip by capillary action. However, since this method requires a flux treatment and cleaning to be performed, the process can become very long, and the environmental control must be enhanced to deal with the problem of disposing of the waste fluid from cleaning. Furthermore, the encapsulation can take a long time due to the fact that it relies on capillary action, thus presenting problems for the productivity.
Therefore, a method of applying resin directly to a circuit board, mounting a chip having solder electrodes directly thereon, and performing solder bonding and resin encapsulation simultaneously has been proposed (see U.S. Pat. No. 5,128,746). This process is characterized by addition of an ingredient having flux action to a resin composition comprising a thermosetting resin and a curing agent for bonding the solder to the circuit board.
More recent proposals have involved preapplying an underfill to the semiconductor chip being carried by a chip carrier and B-staging to eliminate even the underfill applying step during mounting, and preapplied non-flow underfill materials with the additional advantage of providing users in the mounting process with semiconductor chips with underfill.
The aforementioned preapplied encapsulation processes have used only epoxy resins which are solid at room temperature as the thermosetting resin (see U.S. Pat. No. 5,128,746 and JP 2003-212964A).
Additionally, when applying the underfill material to the semiconductor chip in the aforementioned preapplied encapsulation processes, a solvent is added to the underfill material to form a varnish which is then applied and dried. Conventionally, varnishes using a single solvent have been used, these varnishes being good solvents for solid epoxy resins, and being poor solvents for curing agent having flux action (see JP 2003-212964).
However, the conventional art described in the above publications has room for improvement in connection with the following points.
First, preapplied encapsulating resin compositions using only a solid epoxy resin can cause problems such as air entrapment when provisionally placing a semiconductor chip on a circuit board, and this can easily result in voids in the semiconductor package, thus reducing the reliability of the package.
Therefore, one might contemplate using an epoxy resin that is liquid at room temperature to reduce the melt viscosity, but in that case, there can be tack after B-staging, and this can reduce the productivity of the dicing process.
Second, solid epoxy resins often have high crystallinity, and when encapsulating resin compositions containing these are applied to wafers and B-staged, they tend to become brittle and therefore susceptible to damage. For example, when the wafers are singulated by dicing, the blades can chip the B-staged encapsulating resin composition.
Third, while the singulated semiconductor chips described above are bonded to the substrate by thermocompression in the next step, the brittleness of the B-staged encapsulating resin composition can cause damage while handling the semiconductor chip before this second step is reached.
Fourth, since solvents which have good solubility for solid epoxy resins and poor solubility for curing agents having flux action generally have high boiling points, a drying temperature and drying time is needed in order to satisfactorily remove the solvent when applying the underfill material to the semiconductor chip. As a result, a problem arises in that a reaction between the epoxy resin and the curing agent progresses to some degree while the solvent is drying. Additionally, reactions between the epoxy resin and curing agent can affect the subsequent flux activity or remelting of the resin when bonding to the semiconductor chip, or reduce the pot life after B-staging.
Fifth, an adequate time is required to achieve satisfactory solvent removal and drying, thus prolonging the work time for the B-staging process.
Therefore, means for at least partially resolving the problems of the above-described background art have been sought. The means should offer a preapplied encapsulating resin composition in which there is little or no air entrapment. Additionally, the means should offer a preapplied encapsulating resin composition excelling in productivity and reliability. Furthermore, the means should offer a preapplied encapsulating resin composition that is not likely to be damaged by any process.
Additionally, the means should offer a liquid encapsulating resin composition that improves the solvent removal efficiency when the underfill material is being dried after application to the semiconductor chip, shortens the drying time and achieves a long pot life and maintains flux properties for a long time due to shortening of the heating time.
Furthermore, the above means should offer a preapplied encapsulated component and semiconductor device produced using the above-mentioned encapsulating resin for preapplication, and a method of producing the same.